Nuclear measurement devices have been used to measure the density of a medium flowing through a meter. Present devices, however, have many drawbacks, for instance, an inability to make measurements in real-time, a lack of consistency in measured values and a limited range of application, typically liquid flowing through a metal pipe. The devices also come with many safety restrictions. Nuclear meters cannot be transported without proper paperwork, and there are restrictions on the transport of nuclear materials. The dredging industry cannot use nuclear density meters on ships because nuclear sensors are not meant to be moved. Nuclear meters also suffer from being able to measure only a single column of fluid defined by the diameter and longitudinal length of a pipe. The volume measured is necessarily small, and nuclear density meters measure in one spatial direction only. The density of a fluid can, however, vary substantially within the cross-section of a pipe. Nuclear sensors are subject to stringent safety and security standards. Current nuclear density meters have about 80% accuracy with a wait time between measurements of 2-10 minutes.
Other techniques for measuring the density of a continuously flowing medium are known. Ultrasound and microwave sensors, for example, also suffer from being able to measure only a fraction of a cross section of pipe and being limited by a maximum pipe diameter. The signal from these measurement techniques becomes irregular noise above about 15% solids. These measurement devices can have probes that are exposed to the medium making them undesirable for applications involving highly corrosive or abrasive media as one finds, for example, in the mining and dredging industries. Microwave sensors are limited to media with a consistent electrical relative permittivity and a high conductivity.
Auto-sampling has been used to measure the density of continuous flowing media. In this approach multiple samples are obtained throughout a testing period for density measurement in a lab environment. Evaporation en route to the testing facility can occur, however, leading to an overestimate of the percent solids of the slurry sampled. Another drawback to auto-sampling is the wait time. It can take up to 24 h or more to obtain a single reading, which is unacceptably long in many industries. This approach to sampling is also limited to small volumes. This increases the odds the measurement will accurately measure the sample but not the system from which the sample was obtained.
A Coriolis meter has been used to measure the density of a fluid medium in a pipe. Such meters make use of a thin-walled bent pipe. Medium flowing through the bend causes it to vibrate. Measurement of the phase shift in vibration frequency at the end of the bent pipe enables measurement of the mass flow rate. Abrasive slurries like those common to the mining industry erode the bent pipe within weeks, or in some cases days. Another disadvantage of these meters is the small inner diameter of the bent pipe.
Current fluid density measurement techniques display a variety of limitations. They are therefore less useful than desired in industries such as mining, dredging and waste water management. Accordingly, it would be beneficial to these industries to provide an apparatus and methods that provide accurate, repeatable, highly resolved, continuous and real-time sensing and measurement of the density of a fluid medium flowing through a meter and avoid the drawbacks noted above.